Toxic Gas Inhalation Silo Fillers Disease

Farms with large numbers of livestock typically rely on a large storage container called a silo to store animal feed. A variety of relatively airtight structures can serve for animal feed storage, including upright metal tower silos, in-ground pits, and even huge plastic bags. In the silo, recently harvested grains are tightly compressed to squeeze out most of the air. The remaining oxygen is consumed rapidly by actively metabolizing plant cells. As the silo becomes anaerobic, rising amounts of organic acids are formed, resulting in lowering of the pH (acidification) with suppression of microbial overgrowth and prevention of spoilage. As a result, nitrogen oxides (nitric oxide, NO, or nitrogen dioxide, NO2) are generated during fermentation of silage. Nitrogen oxides (NOx) are dangerous chemical gases released from reactions between nitric acid and organic materials. They are severe respiratory irritants of low solubility that penetrate to the lower respiratory tract. When levels of NOx rise in a closed tower silo, the levels of NO2/NOx may rise progressively in the following 1 to 4 days. During this period, the silo becomes a major hazard for any worker without respiratory protection who enters the silo or works in buildings connected to the base of the silo. Fatal exposures by inhalation of silo gas can occur in this setting. Acute high-level exposure can be a cause of acute hemorrhagic pulmonary edema and death. In addition to the potential for exposure to fatal asphyxia secondary to NO2 and other oxides of nitrogen, less severe exposure to nitrogen oxides produces transient pulmonary decompensation, cough, dyspnea, and headaches. Long-term pulmonary consequences can occur secondary to fibrotic scarring (7,8,11,12).

The generation of toxic silo gases can occur unpredictably despite adherence to usual work practices. Although the potential for silo gas formation exists with any type of ensiled feed, the risk appears to be highest with corn silage (8,13).